While you can write a process to first check for the existence / absence of a month of data before an append is performed, that is outside the scope of this discussion group. If I were tasked with auditing your data prior to, or after submission to the FDA, I'd want reassurances that the append processes had worked correctly, without loss or duplication of any data. The problems start with verifying that when a certain month of data is appended, the month is complete and is not added before the end of data collection. Overcoming that limitation adds another level of complexity to the problem and requires a custom built process for your site. Technical Support or a skilled consultant would be needed for this.
It was my understanding that data for clinical trials was managed by a data administrator, whose responsibility it was to ensure that data submitted is complete, in the correct format and only added once. Do you not have a person in such a role managing your data libraries?
Kind regards
David